1. Field of the Invention
The present invention relates to shock absorbers for vehicles. More particularly, the invention relates to fluid dampers. More particularly still, the invention relates to methods and apparatus for controlling and managing pressurized gas used in a fluid damper reservoir.
2. Description of the Related Art
Suspension systems (e.g. “shock absorber”) for vehicles, including motor vehicles and bicycles, include a spring portion and a damper portion. The spring portion creates resistance to a shock absorber's compression, and that resistance may increase non-linearly as the spring is compressed. In some instances, springs are coil springs or leaf springs and in other instances they are gas springs that produce a non-linear compression curve due to the compression of a given quantity of gas. Dampers, on the other hand, produce resistance as a piston moves through substantially incompressible fluid, making the operation of the damper dependent upon shock absorber compression velocity rather than the stroke position of the suspension system. A fluid damper typically involves a chamber having a damping fluid disposed therein and a piston and rod which together move in and out of the fluid chamber as the suspension system compresses and rebounds. The piston is equipped with fluid passages, usually including shims, which restrict the flow of fluid through the piston from one side of the chamber to the other and provide the damping effect as the piston and rod move into and out of the chamber.
When the piston and rod enter the fluid chamber, a volume of fluid equal to the volume of the incoming piston rod must be displaced from the chamber. In order to compensate for the reduction in space for the fluid, a reservoir is used which typically consists of a floating piston that operates with a volume of gas behind the piston. As the volume in the fluid chamber decreases, the floating piston moves against the volume of gas which thereby becomes compressed. In this manner, the reservoir volume available for the damping fluid can increase and decrease during each respective compression and retraction stroke of the shock absorber. A reservoir can be integrally included as a part of the damper chamber or can be a separate chamber, usually adjacent the main damper chamber.
In some instances, especially for larger capacity dampers used with motor vehicles, it is not unusual to have the gas volume of the remote reservoir pressurized for use to 200 psi. Due to the physical size and weight of the damper and the tendency of the charged gas reservoir to extend the piston and rod, these shocks can be difficult to mount when the gas volume is charged with pressure, due to an individual's inability to compress the rod and piston by hand. This problem has been addressed by providing the damper to the customer in a pre-charged condition, but with a strap or other temporary restraining member retaining the damper in a partially compressed position. After the damper is installed on a vehicle, the strap is removed to permit the damper to extend to its fully extended position.
Because the gas pressure of the gas volume in a damper affects the overall performance of the damper, maintaining appropriate gas pressure is important. In some instances, however, the mere act of checking the pressure results in a gas volume which is under-pressurized due to the small gas volume and the necessity of utilizing at least a small part of the gas while checking the pressure. Constant checking only exacerbates the issue and often results in an underpressured reservoir.
What is needed is a way to facilitate the installation of dampers on vehicles while ensuring that the damper will be provided with the appropriate gas pressure in the reservoir portion. Additionally, there is a need for a safe and easy way to determine whether a gas volume in a damper is appropriately charged with the desired amount of gas pressure.